SIP Trunk

The connection to the Emergency Services Service Provider is accomplished through a dedicated SIP trunk. Emergency providers can only accept calls from a single Mediation server, so if a pool of Mediation servers exists already, an additional, standalone Mediation server must be deployed to support E911 to the Emergency Services Service Provider.

To provide resiliency for E911, an additional SIP trunk to a different Mediation server must be provisioned. This ensures that if the primary SIP trunk or Mediation server is unavailable, E911 calls can still be delivered correctly. This additional trunk can be in the same location or, ideally, in a different site, to ensure site survivability. When provisioning the SIP trunk, a VPN tunnel to the Emergency Services Service Provider is created using an existing Internet connection, or a dedicated connection can be provisioned to separate and isolate the emergency calls.

The final consideration with the SIP trunk is to recognize it does not bypass Call Admission Control policies. If a bandwidth policy is exceeded by an emergency call, the call will not succeed. When planning for E911, be sure to consider the effects of Call Admission Control on where SIP trunks to an Emergency Services Service Provider are placed. For example, in a site where WAN bandwidth is constrained, it makes sense to deploy a local Mediation server and SIP trunk to a provider to ensure Call Admission Controls never prevent an emergency call across the WAN link.